Intramolecular, addition photochemical

Other nucleophiles add to conjugated systems to give Michael-type products. Aniline derivatives add to conjugated aldehydes in the presence of a catalytic amount of DBU (p. 488). Amines add to conjugated esters in the presence of InCla, La(OTf)3, or YTb(OTf)3 at 3kbar, for example, to give P-amino esters. This reaction can be initiated photochemically. An intramolecular addition of an amine unit to a conjugated ketone in the presence of a palladium catalyst, or... 

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

Photoelimination of nitrogen from the tetrazole 429 results in the formation of the dihydropyrazole 430, presumably via intramolecular addition of the photochemically generated nitrile imine 431.358 Other examples of this type of behavior have been reported.359... 

Intermolecular addition of photochemically generated nitrenes and in particular acylnitrenes to alkenes provides a useful and widely applied route to aziridines.385 An analogous intramolecular photoreaction is thought to be involved in the conversion of the o-azidophenylethylfuran 461 into the pyrrolo[l,2-a]quinoline 462 as outlined in Scheme 13,386 and intramolecular addition to an azo group has been observed in the 8-azido-1-arylazonaphthalenes 463.387... 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

The most widely exploited photochemical cycloadditions involve irradiation of dienes in which the two double bonds are fairly close and result in formation of polycyclic cage compounds. Some examples are given in Scheme 6.7. Copper(I) triflate facilitates these intramolecular additions, as was the case for intermolecular reactions. 

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

Intramolecular addition of heteroatom radicals to olefins constitutes a convenient method for the synthesis of heterocycles. The photochemical ring closure reaction of oxyl radical derived from 44 provides access to tetrahydrofuran 45 [95JOC6706]. The regioselectivity in this cyclization is excellent, however, the stereoselectivity is only modest. The stereoselectivity was dependent on the temperature of the reaction. 

Kinetics of the photochemical dimerization of olefins to cyclobutane derivatives. I. Intramolecular addition. J. Amer. chem. Soc. 84, 4141 (1962). 

Alkenyl as well as alkyl nitrenes can be produced both by thermolysis and photolysis. However, photolysis is preferred due to the more favorable reaction conditions such as low temperature and the ability to use of particular solvents, which avoid side reactions such as the polymerization observed under thermolytic conditions. However, in some cases, alkenyl nitrenes can only be generated by photolysis [14]. Photochemically generated alkenyl nitrenes normally react by intramolecular addition to the 7r-bond forming 2H-azirines at rather high yields (Sch. 3) [6d,15], in addition, the... 

Among the variety of terpenes which were synthesized with an intramolecular meta photocycloaddition as key step, triquinane derivatives were particularly well studied. Two isomers of these compounds are readily accessible via intramolecular meta cycloaddition in position 1,3 (Sch. 9). The following cyclopropanation reaction of intermediate O controls which of the angular or linear isomers is formed (Sch. 17) [67]. As this step is almost always unselective, both isomers are concomitantly formed. However, one of the isomers can be obtained predominantly via an additional photochemical equilibration step (compare Sch. 11). The triquinane frame is obtained by rupture of the distant C-C bond of the cyclopropane in connection to the 10-membered ring moiety. 

A slightly different approach has been utilized for the three-carbon expansion in a system which does not contain a carboalkoxy group. The initial intramolecular addition step is carried out separately with butyllithium or samarium diiodide to give a bicyclic alcohol (Scheme 78). The radical-induced fiag-mentation step is carried out photochemically in the presence of mercury(II) oxide and iodine to provide the expanded iodo ketone. 

Intramolecular Additions - The predominant photochemical reaction of the allene derivatives (65) is (2 + 2)-cycloaddition yielding the housanes (66). The reaction occurs in a variety of solvents (e.g. hexane, acetonitrile or acetone) and the triplet excited state of (65) is implicated. In hexane and... 

The unsaturated 6-lactone (84) undergoes intramolecular photochemical (2 + 2)-cycloaddition to give (85)." The scope of the process has been evaluated and the 5-lactones (86-88) all behave similarly, affording the products shown in Scheme 2. The lactones (86) and (87) both cyclise in the two possible modes. Intramolecular cycloaddition reactions within polymethyldia-minebis(4-methyl-7-coumarinyl)oxyacetamides have been described. Zhu and Wu have reported that a biscoumarin system linked by a phenanthroline unit does not undergo photodimerisation. Instead, intramolecular addition of a coumarin unit to the phenanthroline occurs. 

S—C Bond heterolysis dominates the photochemical reactivity of the keto tosylate 263247. Irradiation in benzene gives p-toluenesulphonic acid in 74% yield. The other products formed from this reaction are the ketones 264 and 265. The reaction is presumed to proceed via the intermediate carbocation 266 formed by S—C bond fission. The tosylate 267 is more able to undergo intramolecular addition due to the electron-donating methoxy group and gives 268 and 269 in 24% and 23% yields. An analogous mechanism is involved in the conversion of the sulphonate 270 on irradiation in benzene into the two products 271 and 272 in a ratio of 4 1249. The formation of the major product 271 presumably involves the heterolytic fission of an O—C bond to afford a cation which... 

The final step in the synthesis of 4-thia-5j3-cholestane (279) involves stereospecific intramolecular addition of the photochemically generated thio-radical to the double bond.810 An analogous cyclization must be responsible for the formation of 1,1-dimethyl-1-sila-5-thiacyclo-octane (280) from hydrogen sulphide... 

The regiochemistry of photochemical 2 + 2 cycloaddition is reminiscent of the intramolecular addition of radicals onto an ethylene double bond and especially of the regioselectivity observed in the cyclization process of 5-hexenyl radicals [108] (Scheme 18). The empirical rule of five states that the preferred regioisomer involves a five-membered ring biradical as intermediate [109]. 

De Keukeleire and co-workers have reported a further example of the intramolecular addition of a pyrimidone to a pendant benzene ring. " In this example the cyclobutane adduct (189) formed from (190) on irradiation could not be isolated. Instead it underwent rearrangement on attempted isolation. Cyclodextrin is often used as a template upon which specific photochemical reactions... 

Intramolecular Additions to Cyclohexenones and Related Systems. 3-Styrylcyclohex-2-enone undergoes photochemical cyclization to afford 2,3-dihydro-[177]-phenanthrene-4-one. Work carried out on the photostability of curcumin has also examined the photolability of the chalcone (38). This undergoes cyclization to (39) on irradiation. ... 

The 1980s witnessed a revival of interest in synthetic photochemistry, largely based on the successful application of intramolecular arene-olefin and enone-olefin photochemical cycloaddition reactions to the synthesis of polycyclic natural and unnatural products. The potential application of intramolecular arene-amine addition to the synthesis of alkaloids was suggested in 1973 by Btyce-Smith et al.," based on their observation of intramolecular photochemical 1,4-addition reaction of the (N,A/-dimethylaminoalkyl)benzenes I and 2 to yield 3 and 4, respectively. Intramolecular addition reactions have subsequently been observed for a large number of (aminoal-... 

In the case of monosubstituted cyclobutenone 31, the adduct with lithiovinylsuofone 32 was reported to undergo an extraordinarily facile tandem 47r-67T electrocyclic process (33 34) at - 78 °C to give cyclohexenone 36 [70]. The photochemical process may oblige the opposite direction on a hydroxyl group to be oriented inwardly actually cyanohydrin 37 was reported to give butenolide 39 as a result of an intramolecular addition reaction of (Z)-hydroxyvinylketene 38 [71] (Scheme 6). 

The photochemical behavior of o-allylanilines such as 88 has been studied. Such compounds cyclize to give the (Cy5) compounds, indolines (89) (Scheme 45). This result is clearly reminiscent of the photochemical behavior of o-allylphenols (Section VIII.3), and although the selectivity is the one expected from aminyl radical intramolecular addition, it also seems best explained by photochemical excitation of the n double bond. This is an interesting conclusion since metal-salt-induced cyclization of compounds such as 88 generally yields a mixture of (Cy5) and (Cy6) products. 

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